Self discharge has an _extremely_ strong function of temperature.
http://batteryuniversity.com/learn/article/elevating_self_discharge
It is also a function of cell health, age, past abuse, etc. The list of
factors that alter the rate of self-discharge is seeming endless.
Because it is such a strong function of temperature, small variations in
the temperature of each the many cells in a high voltage pack can cause
large imbalance because "self-discharge never sleeps". It chews away on
the cells 24-7, regardless of whether they charging or discharging or
simply open circuit. This is a problem because the end cells (that have
a thick conductor to the outside world,) and cells on the outside edge
of the pack, have a different thermal environment than do the inner-most
cells. The temperature of the outside cells (and the end cells) is often
starkly different than the inner cells. The self discharge is thus
greatly different, and is typically dependent on the placement of the
cell within the pack, and the difference between the outside temperature
and the average pack temperature.
If your BMS happens to have some sort of cell voltage monitor or some
sort of LED indicators on the cells themselves, this spacial imbalance
becomes readily apparent. You can literally see the temperature
variation that manifests itself as a SOC imbalance across the pack. You
can watch the LEDs on perimeter of the pack light up before the LEDs on
the inside of the pack when the outside temperature is cooler than the
pack temperature. When the outside is warmer than the pack, the opposite
is observed. It is like a topographic map of the cell temperature since
the last charge. Even if the cell temperature is uniform at the time you
actually charge and observe, the BMS LEDs will tell the tale of the cell
temperature history since the previous full charge.
What is particularly insidious, is that contact resistance of the
terminals, and internal resistance, also greatly effect the temperature
of individual cells and thus elevate the self-discharge of those
specific cells. This is why bad connections cause chronic cell imbalance
and "weak" cells get out of balance. These cells run hotter than the
rest, and the self-discharge skyrockets.
Bill D.
On 8/8/2017 6:23 PM, Hoegberg via EV wrote:
Hi
LFP:
You might with some(all?) LFP even find a slight hysteresis in pack voltage, at
exactly the same SOC..
(most visible if you are in the 30-70% SOC-zone)
depending on ..if you have had a regen or a discharge pulse as your last event,
then the no load voltage seems not to be exactly the same, at the same SOC.
A higher rest voltage if you did a charge/regen pulse compared to if you just
did a very short discharge.
I agree with the others, count Ah is the way ot go to know the SOC % in the
flat part of the discharge curve,
Also my experience was, that decent cells dont have any / a lot of self
discharge to balance out when in normal use, only milliamps might be needed
over time, so if they are well (top)balanced once they seems to stay well
balanced. But if the cells are damaged / have mfg problems from the begining
then it might be a different situation,
Regarding balancers maximum current:
we had a 5 Amp as the charger minimum current, so we did a pulse charge
instead of use large balance currents,
So if one cell reach the "balancing" voltage then we can just stop the charger,
and wait for that cell to reach its lower voltage, with only 100mA or so as balancer
discharge current, then we re-enable the charger(5Amp) until any cell(s) again reach the
balance-start voltage.
If you dont have any cell voltage monitoring , or any kind of signal / feedback
from the balancers, then it might be tricky to do this, I dont have any good
solution to shut of the charger in time if we dont know when we have a problem.
(other than to use a lower charge current than your balancers can handle, but
if one balancer do fail, then you will probably overcharge that cell later)
I would prefer to use some kind of good cell voltage monitoring so you can
get a warning in time if some cell go to low or to high, and also use it to
shut of/cut down the charger, or cut back on the trottle if some of the cells
get to low when driving.
in my opinion that should be a minimum when charging a large expensive pack..of
more than 4 cells in series. :-)
If we only use the full pack voltage for the charger to decide whan to go in to constant
voltage mode, then we can get in troubles, for example if one cell in the pack reach
"full" and lift off almost like a capacitor, long before all the others have
start to climb up faster in the end, so if all the other cells that still are the flat
and lower voltage region the charger will give the pack and the already full cell its
maximum current. Not good.
For example:
if we use 3.60 V as the chargers maximum cell voltage * 25 cells = 90 volt
what now if one cell is full and the others are still at 3.45 V each?
3.45 * 24 cells = 82.8 volt
Minus..say..89.8 Volt from the charger?
= 1 cell will now try to reach up to about 7 Volt, and maybe still at full charger
current...if so, that can probably be "bad". :-)
/ John
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